Republicans and Democrats in Congress are trading blame ahead of Tuesday’s government shutdown, but there’s another culprit in DC’s latest dysfunction whose offices are not to be found in the gilded suites of the Capitol, but in a drab, fluorescent-lit office five blocks away. There, a team of organizers, lobbyists and 20-something social media specialists are harnessing the power of the Tea Party to drive a wrench into Congress’ gears. Heritage Action for America, the political arm of the once esteemed Heritage Foundation, has been working day and night for years to bring about just the crisis now gripping DC.

Since its founding in 2010, Heritage Action has worked aggressively to influence lawmakers on issues from immigration to agriculture to the budget. It uses a three-pronged strategy to twist arms on Capitol Hill: lobbying members on hot-button issues, ranking them publicly on how they vote, and getting word out far and wide when lawmakers buck the conservative line. That combination—unparalleled in either party—has given Heritage the ammo to take on the leadership of its own party, widening the gap between the conservative grassroots and their leaders in Congress. As Heritage’s clout accumulates, the group’s divisive style offers a window into the new way of doing business in Washington, where, thanks to redistricting, lawmakers are more worried about primary challengers than the opposition.

Take the fight over President Obama’s signature first term legislation, the Affordable Care Act. Republican leaders who survived the politically disastrous budget battles of 2011 hoped to avoid a similar fight as deadlines for funding the government and raising the debt limit approached this fall. Then Heritage Action threw their weight behind a long-shot effort to defund Obamacare, on the simple math that to its Republican supporters, the healthcare law is viewed as more toxic than a government shutdown.

Led by Mike Needham, the group’s CEO, Heritage launched a nine-city bus tour and recruited Tea Party icon Sen. Ted Cruz of Texas, and his father Rafael Cruz, a celebrity in conservative circles, to back the cause. “Politicians don’t lead, politicians follow,” Needham told a crowd of more than 700 in Tampa. “They need to be forced by you all to do the right thing. They won’t do the right thing by themselves.” Forcing the GOP to act became the group’s solitary mission for the ensuing weeks. Heritage Action poured $550,000 into a digital advertising campaign targeted at Republican congressional districts and worked its social media base to muscle GOP lawmakers into supporting the measure. “It’s tremendous pressure,” says Kansas Rep. Tim Huelskamp, of Heritage’s influence on lawmakers.

By the time Congress returned in September, scores of House Republicans were on board, despite pleas from party leadership, which regarded it as a kamikaze effort. Needham further bolstered his ranks when he announced three weeks ago that Heritage would rank lawmakers on their votes on a procedural motion allowing for the continued funding of Obamacare. Wavering GOPers quickly joined the opposition and House Speaker John Boehner abandoned his plan to push for a temporary, rather than permanent, defunding of the health care law. “Without their help we probably wouldn’t have gotten it done,” says Arizona Rep. Matt Salmon.

Heritage’s willingness to take aim at its own party has irked more mainstream Republicans. “When you’re spending all of your money attacking Republicans and not the Democrats who voted for Obamacare, it’s entirely fair to question what your motives are,” says Brian Walsh, a Republican consultant and the former communications director for the National Republican Senatorial Committee. ‘They’ve been effective at advancing their own power in Washington and at the cost of giving Democrats a free pass.”

But Heritage sees themselves as having a higher calling than party politics. “We’re not ‘good Republicans,’” says Heritage Foundation Executive Vice President and Chief Operating Officer Phillip Truluck. “We’re conservatives.”

House Republican leadership aides have been quick to offer criticisms of the group on the condition of anonymity, while some representatives have publicly taken on Heritage. “Why is @Heritage_Action spending $550K to attack conservatives but not @KayHagan who was a deciding vote on #Obamacare,” tweeted Rep. Renee Ellmers (R-N.C.) last month. “Should we stop #Obamacare? YES! But @Heritage_Action’s strategy w/ Continuing Resolution is WRONG.” “I just want to go back to fighting Democrats,” said one Republican leadership aide.

As Washington stumbled toward a shutdown this weekend, Heritage Action has pledged to include votes on a “clean” continuing resolution in its running scorecard, even if the government shuts down. The group wouldn’t fully support the House Republican measure to delay Obamacare by a year as part of a measure to keep the government open, but in a small concession to Republican leadership, it also didn’t oppose Saturday’s House vote.

“Although it falls short of full defunding, Heritage Action will not key vote against the Blackburn Amendment because a delay is a step towards preventing the law’s entitlements from taking root,” the organization said in a statement, referring to tallying the votes of lawmakers on important bills for the purposes of their conservative ranking.

In the Senate too, Heritage Action, along with other conservative groups like the Club for Growth, has managed to make its mark. Senate Minority Leader Mitch McConnell and Minority Whip John Cornyn are both up for reelection in 2014 and are increasingly wary of conservative primary challengers.

There is dissent about combative strategy inside Heritage Foundation as well. In the initial years of the group, management grew concerned by Heritage Action’s free-wheeling style, moving them from a town house nearby into the fourth-floor of its headquarters building. As Heritage Action flourishes, Heritage Foundation has become less of a force on Capitol Hill, its role for three decades as an omnipresent voice in hearing rooms and Senate hideaways crafting legislation for Republicans now firmly in the rearview mirror.

A number of the think tank’s top researchers have decided to leave the group in recent months over the controversy caused by Heritage Action, according to people familiar with the situation. “That place is hemorrhaging,” said one former employee. The selection of former Sen. Jim DeMint to run the think tank last year has confirmed the organization’s commitment to political activism. DeMint, who made a name for himself in the Senate by supporting conservatives in primary contests—even primarying his GOP colleagues—leading to victories by Ted Cruz and Marco Rubio, but also nominations for two of 2012’s biggest losers, Richard Mourdock and Todd Akin. The group fiercely denies that its research arm has been harmed by the political side.

Such criticism means little to Heritage Action. Despite Needham’s best efforts and Cruz’s 21-hour speech against it, the online insurance marketplaces that are a centerpiece of Obamacare will debut as planned Oct. 1. Republican leadership fears it will bear the blame for government shutdown, endangering vulnerable Republicans in 2014. But for Heritage and its allies, the resonance of their message is more than a moral victory. “I think that this campaign has already been very successful in the sense that we’ve driven the narrative in the last two months about how Obamacare is literally falling apart,” Needham says. “There’s been huge national attention in the media and the grassroots. That’s a great thing for the country and a great thing for us.”